Electric starters for internal combustion engines often employ a pinion gear which is axially displaced upon the motor drive shaft for selective engagement with gear teeth defined on the engine flywheel. While various devices and mechanical elements have been used to displace the pinion gear upon the motor armature shaft it is commonly known to use helices formed upon the shaft which engage with a nut threaded thereon to axially translate the pinion into engagement with the flywheel teeth. With electric starters for small internal combustion engines such as found on snow blowers, lawn mowers, garden tractors, and the like, rapidly rotating electric motors are used wherein the initial resistance to rotation of the nut member and associated structure upon energization of the motor is used to axially displace the nut member and pinion for engagement between the pinion gear and flywheel. Such operation results in rapid axial displacement of the pinion gear, and unless the gear and flywheel teeth are properly aligned the pinion gear will engage the side of the flywheel gear until alignment occurs, resulting in flywheel or pinion gear tooth peening which, over a period of time, may cause a gear tooth to deform, fracture, or bind with the mating teeth.
In order to cushion the initial engagement between the pinion and flywheel gear teeth combination an elastomeric cushioning and torque transmitting member may be interposed between the nut and pinion gear to cushion and absorb the impact between the pinion gear and misaligned flywheel tooth, and the cushioning member may also be used to transmit the cranking torque from the armature shaft through the nut and to the pinion gear. Thus, the elastomer cushion will absorb torque vibrations during cranking, as well as cushion the initial engagement, and aid in the alignment of the pinion gear and flywheel gear teeth.
Small internal combustion engines often employ aluminum flywheels utilizing gear teeth formed of the same material, and as the starter pinion gear may be formed of steel the flywheel gear teeth may be damaged from repeated impact by the pinion gear if the pinion and flywheel gear teeth are not properly aligned as the pinion gear enters the flywheel teeth. To minimize damage between the flywheel and pinion gear teeth the assignee has developed cushioning members capable of producing an initial "soft" cushioning of the pinion gear upon initial engagement with the flywheel teeth, and as the torque requirements increase a stiffer or firmer cushioning is achieved which is capable of transmitting the desired torque. Electric starters produced by the assignee have utilized various elastomeric cushioning members, and examples can be found in U.S. Pat. Nos. 3,791,685; 4,330,713 and 4,347,442.
The elastomer cushioning member of the aforedescribed type is usually of an annular configuration and circumscribes the helices formed on the motor armature shaft. As the elastomer material is highly compressed during cranking and will deform radially one common problem arises from the tendency for the elastomeric material to extrude into the shaft helices wherein elastomer particles become trapped within the helices and cause the nut to bind with respect to its movement on the shaft.
It is an object of the invention to provide an electric starter for internal combustion engines utilizing an elastomeric cushioning and torque transmitting member wherein a pinion gear is employed having a recess receiving an annular ring defined upon the elastomeric member wherein the pinion gear recess partially confines the elastomeric material during cushioning and torque transmission.
An additional object of the invention is to provide a nut, pinion gear and elastomeric cushion assembly for an internal combustion engine electric starter wherein all three components are mounted upon a starter shaft, and the pinion gear and elastomeric cushion are provided with interrelating concentric configurations which cooperate during engine cranking to concentrically maintain the cushion upon the starter shaft.
An additional object of the invention is to produce an electric starter assembly for internal combustion engines utilizing helices defined upon the starter motor shaft wherein the helices are of a greater helical angle than is the common practice in order to produce engagement between a pinion gear and the engine flywheel before the starter shaft reaches its maximum rate of rotation, and thereby reducing the degree of impact between the starter pinion gear and flywheel gear teeth in the event of tooth misalignment.
In the practice of the invention the electric starter motor includes an armature shaft which extends from the motor housing having a free end upon which an abutment is defined. A helical thread of heavy duty type, such as of square configuration, is defined upon the armature shaft, and in the disclosed embodiment is adjacent the free end. The helices preferably have an unusually high angle, preferably approximately 33.degree. , as compared with the usual helical angle of approximately 23.degree. with this type of starter.
A pinion gear is rotatably mounted upon the armature shaft having a smooth bore for axial as well as rotational movement thereto, and the pinion gear includes a radial friction surface having an axially extending annular groove or recess intersecting the friction surface and forming a part thereof. The gear groove is concentric to the shaft axis and is defined by inner and outer conical surfaces converging toward a base.
A nut member, in the form of a flat plate or washer, is mounted upon the shaft helices, and includes a threaded bore to produce a mating and threaded relationship with the helices and a radial friction surface is defined on the nut. Thus, relative rotation between the nut and shaft will produce an axial displacement of the nut upon the shaft.
An elastomeric cushion and torque transmitting member of annular configuration is interposed between the friction surface of the pinion gear and the flat friction surface of the nut. The elastomeric member includes a radial surface engaging the nut friction surface and complimentary in configuration thereto. The elastomeric member also includes an axially extending ring projection which is concentric to the armature shaft and extends toward the pinion gear. The ring projection is formed by conical inner and outer surfaces which converge toward a nose which is in radial alignment with the pinion gear groove and received therein. The included angle defined by the pinion gear groove surfaces is greater than the included angle defined by the elastomeric ring surfaces whereby a clearance exists within the gear groove between the groove and elastomeric ring material until deformation of the ring material occurs. As the amount of elastomeric ring material at the ring nose is relatively small, and as the ring material may radially deform into engagement with the gear groove during initial stages of pinion gear displacement and cranking, an initial "soft" cushioning of the pinion gear is provided, and as the axial forces imposed upon the elastic member by the nut increase a greater amount of elastomeric material is placed under compression, "stiffening" the cushioning characteristics of the elastomer and permitting the necessary torque forces to be transmitted between the nut and pinion gear.
As the pinion gear groove is concentrically oriented to the armature shaft the reception of the elastomer ring into the groove will aid in centering the elastomer relative to the shaft and maintaining it concentric thereto while the elastomer is under compression and deformed. This support of the elastomer aids in keeping the elastomer from entering the helix, and minimizes the likelihood that elastomeric particles will enter the helix and interfere with the nut movement thereon.
A compression spring circumscribing the armature shaft biases the pinion gear in an axial direction toward the nut member and elastomeric cushion, and a stop cup mounted upon the shaft functions to position the pinion relative to the flywheel during cranking.